1. Field of the Invention
The present invention relates to a high resolution fiber optic position sensor system. Specifically, the system is comprised of a remote sensor head connected by a pair of optical fibers to an electronic controller unit. The sensor is an electronically passive device that utilizes optical signals only. It senses the angular absolute position over the full 360° rotation with a sensing resolution of 13 bit (8196). The sensor modulates the optical signal based on the exact position of the sensor disk. This modulated optical signal is analyzed in the controller and translated into the position signal. Because the sensor head is electrically passive it can be deployed in an EMI/RFI intense environment. Further, it is immune to lightening strikes and can also be located many hundred meters away from the control unit without creating undesirable ground loops.
The invention disclosed herein is unique in that the position information is embedded in the optical spectrum rather than in the optical amplitude. This is significant because attempts to make use of the optical amplitude (signal strength) are unsuccessful because amplitude levels in optical fibers are unstable and typically vary well beyond 10%. Because the optical spectrum cannot vary and will not vary even when the amplitude changes, the instant invention is highly accurate and insensitive to changes in optical signal variation in the fiber.
2. Description of the Prior Art
The need for accurate position sensing is everywhere. Nearly anywhere mechanical motion is effectuated by means of an automatic system, a sensor to measure position is used. Position sensors are found in applications such as industrial automation, CNC machining, transportation, aviation, shipboard, trains, medical imaging, cranes, wind turbines, radio telescopes and the like.
Position sensor technology is generally regarded to be mature and incremental refinements are being regularly made. The field can be broken down into a few fundamental technologies. These include optical incremental encoders, optical absolute encoders, electromagnetic resolvers (inductive), potentiometers (varying resistance) and magnetic field sensors (Hall effect). All the above listed sensors process an electric current. They need to have an external electric power source or internal battery and typically require the connection of electrical wires to a control unit.
In many applications the electrical connection between sensor and control unit is cause for problems. External electric fields may induce noise in the transmission lines causing erroneous position information or, in the case of lightening, may even destroy the sensor or control unit.
Position sensors are also often used in petro-chemical processing to sense the position of valves, gates covers, baffles etc. In these environments electrical wires may cause sparking and are considered hazardous items.
Thus, the present invention describes a position sensor that measures the position by means of optical light signals only. By eliminating any electrical components, this new sensor becomes immune to any electrical interference whatsoever.
Rotation sensing and linear measurement techniques are numerous and many varying concepts have been invented and used. Described herein are concepts for measuring absolute position. In defining absolute position, one needs a sensor that will report an accurate position at all times, after initial power up and without any mechanical movement present on the sensor. Absolute sensors do not require an initial calibration or homing after the system power has been applied.
Absolute encoders, or position sensors, are ubiquitous everywhere in our daily lives. Described herein are the fields wherein there is a foresseable need for a passive non-electronic position sensor. One example includes chemical and petrochemical processing because of the need to eliminate any electrical arcing.
The instant invention is applicable to the field of transportation, and specifically with regard to electric trains where position sensors may be utilized on the pantographs. Voltage line position sensing can also benefit from the instant invention.
Wind turbines would benefit because the sensor is immune to lightening and atmospheric discharge.
Because the sensor is immune to EMI/RFI (directed energy weapons), there are military applications as well.
Medical deployment within radiation and high magnetic fields such as MRI machines can also benefit. The sensor can be built entirely from non magnetic materials and thus is immune to any external interference and will not disturb any magnetic field.
In shaft position feedback applications for vector drive motors, these are relatively slow turning motors where the magnetic vectors are entirely controlled by an external computer and based upon precise shaft position location. A fiber optic position sensor may be useful because it will not be affected by the high magnetic fields of the motor windings.
In general feedback devices (position sensors) are connected electrically (or lately wirelessly) to a controller a certain distance away from the sensor. They are all plagued by EMI/RFI interference and ground loop issues. Of course, established techniques mitigate these issues but still the problems are a constant and systems integrators deal with them on a daily basis. Some prior art methods are described below.
Optical absolute sensors obtain position information by reading a digital code from a rotating disk. The disk has a number of tracks engraved with a binary code. Optical detectors read the track information and translate it to a position signal. These encoders incorporate the rather complex electronic components in close proximity to the rotating disk. Although very accurate and very high resolution, these encoders tend to be fragile and susceptible to electrical and environmental interferences.
A resolver is by nature an absolute sensor. It is a rotary transformer where the magnitude of the energy through the resolver windings varies sinusoidally with the rotation angle of the shaft. These sensors must be energized by one or two alternating voltage sources and will return position information as an AC signal with varying amplitude and phase. No electronic components need to be present in the sensor and thus it makes these sensors more robust mechanically and against electrical interferences. However, resolvers must be connected by electrical wires to the control unit and interfering signals may disturb or even damage the control unit.
Magnetic Hall effect sensors consist of a semiconductor material whose electrical resistance varies with the strength and orientation of a magnetic field. When a Hall effect sensor is exposed to a magnetic field such as a permanent magnet the magnetic field orientation can be determined based on the resistance of the semiconductor.
A number of patent innovations have attempted to solve the issue with a fiber optic encoder, or position sensor. Numerous inventions have been recorded for an electronic passive position sensor. A few patents take the “brute force” approach by simply stringing a number of optical fibers and read tracks of a binary code disk. This is not a practical solution as a high resolution sensor requires at least 10 bits and thus would require at least 10 optical fibers connecting to the sensor. Because the complexity is very high, the reliability is decreased.
Other methods use orthogonal polarization methods where a rotating disk alters the polarization of the light. This approach is technically complex and requires polarization maintaining fibers (PMF) to be deployed. Even so PMF fibers are susceptible to environmental influence such as temperature and mechanical stress that alter the state of polarization. Ultimately there is only limited accuracy that can be obtained by this scheme.
Methods using WDM (wavelength division multiplexing) come closer to being practically realizable. U.S. Pat. No. 4,849,624 describes such a system. This system fundamentally could be reduced to a practically deployable unit. However, the described encoder lacks simplicity because it utilizes concentric tracks for which each wavelength component must be accurately aligned with said track. As higher resolution is demanded, the dimension of the track becomes increasingly small and the optical focal point must be accordingly small and narrow and positioned accurately. The end result is that the optic system requires incredible precision. It is not feasible to build an encoder in excess of 10 bits that also works reliably.